Method for manufacture of isoprene



May 23, 1951 J. F. MAGNESS METHOD FOR MANUFACTURE oF IsoPRENE FiledApril 30, 1958 INVENTOR JAMES F. MAGNESS ATTOR Y www United StatesPatent C METHOD FOR MANUFACTURE OF ISOPRENE James F. Maguess, Tulsa,Okla., assignor, by mesue assignments, to Standard Oil Company, Chicago,Ill., a corporation of Indiana Filed Apr. '30, 1958, Ser. No. 732,044

3 Claims. (Cl. 260-680) The present invention relates to the manufactureof isoprene. More particularly it is concerned with the preparation andrecovery of isoprene from a readily available, relatively cheap sourceof supply.

The process of my invention is carried out in two dehydrogenerationsteps. First a C5 fraction obtained, for example, from natural gasolineor an equivalent source, is subjected to a combination dehydrogenationand isomerization treatment. The resulting mixture is next selectivelyextracted to separate the Z-methylbutenes, thus formed, from theremainder of the components present in the product mixture. Thereafter,the stream of Z-methylbutenes is introduced into a seconddehydrogenation zone where said butenes are converted into thecorresponding diolein, i.e., isoprene.

Recently it has been discovered that polymers having characteristicsvery similar to those of natural rubber, can be made by polymerizingdioleiins, such as isoprene, in the presence of certain catalysts havingthe ability to form high molecular weight polymers containing a highpercentage of specific isoprene stereo-isomers. Such catalytic materialsare usually referred to as stereospecic catalysts. This discovery hasvery appreciably increased the importance of providing industry with apractical and eilicient method for the manufacture of isoprene.Presently the production of isoprene is limited to manufacture fromrefinery gases and other small Volume sources. Moreover, the isopreneobtained lfrom these sources is diiiicult to recover because it is foundin the form of a mixture, together with some ten or twelve other C5hydrocarbons, all of which boil over a range of about 30 C. Six of thesehydrocarbons boil within less than C. of the boiling point of isoprene.Consequently, while isoprene can be recovered from such a source in arelatively pure condition, the number of steps and equipment requiredybring the cost up to a level which materially decreases the economicappeal of the above-mentioned new polymerization process,notwithstanding the desirable properties of the polymers made thereby.

Other procedures currently of interest for the preparation of isopreneinvolve the catalytic dehydrogcnation of isopentane. Actually,production of isoprene from n-pentane is more desirable since the costof isopentane is substantially greater than that of n-pentane or ofmixtures of isopentane and n-pentane, such as occur in natural gasolinefractions. Production of isoprene from the normal isomer, however, iscomplicated by the fact that on dehydrogenation of n-pentane a largenumber of C5 unsaturates, other than isoprene, are produced, thusrequiring an expensive separation procedure for the recovery of thedesired isoprene. As a matter of fact, even when using isopentane as thestarting material, a substantial amount thereof is converted toundesirable derivatives through isomerization.

Accordingly, it is an object of my invention to provide a method for themanufacture of isoprene from a readily available cheap large volumesource under conditions which involve first isomerizing anddehydrogenatng a C5 fice hydrocarbon rfeed stream, recovering theZ-methylbutenes thus produced, and thereafter subjecting saidr butenesto a yfurther dehydrogenation step to producethe desiredisoprene.

My invention will be further illustrated by reference to theaccompanyingflow diagram, wherein a C5 fraction obtained from naturalgasoline, Yfor example, and consisting essentially of n-pentane andbranched chain C5 hydrocarbons, including isopentane, is introducedthrough line 2 into unit 4 in which these hydrocarbons are convertedinto olelins and isomerized. Whether isomerization occurs iirst followedby dehydrogenation ory whether the two reactions occur simultaneously isunimportant insofar as the present invention is concerned.

The unit or reactor 4 contains, as a catalyst, alumina or an oxidederived from metals of groups IVB, VB or VIB of the periodic chart ofthe elements. The lastmentioned metal oxides'are preferably deposited ona suitable support such as, for example, silica, silicaalumina, etc.Usually, the oxides of chromium, molybdenum and vanadium deposited onalumina, are preferred as the dehydrogenation components of my catalyst.While the isomerization component may consist of platinum oxide orplatinum on a support, such as one of those mentioned above, othersuitable isomerization catalysts may be prepared from a compositecomprising a calcined precipitate of hydrous silica and hydrous alumina.Also, isomerization catalysts which have, as their chief components, analkali-free hydrous silica, may be used. Additionally, hydrous zirconiamay be combined with hydrous silica to form the active components of theisomerizationl catalyst employed. Such materials are highly active andthermally ystable at the temperatures encountered. These catalysts maybe prepared by forming a composite mass of silica hydrogel and aluminaor zirconia hydrogel, followed -by drying and calcining at 800 to 1,500F.

The temperature range employed in this combinationdehydrogenation-isomerization step, may in general range from about 900to about 1,l00 F., preferably from about 950 to about `1,050 F., atapressure of from about 2 to about 4 p.s.i.a.

From the dehydrogenation and isomerization unit 4, the mixture ofconverted and-unconverted material is taken olf through line 6,condensed and the resulting liquid sent to extractor 8, where saidmixture is countercurrently contacted with a stream of a selectivesolvent for 2-methylbutenes, such as sulfuric acid of about 60 to about75 weight percent strength and preferably in a concentration of fromabout 65 to about 70 weight percent. Under these conditions aregenerable complex is formed by reaction of sulfuric acid with thebranched chain oleiins in said mixture. The extraction step maybeeffected from about 50 up to about 100 F. While the reaction involved inthe formation of the aforesaid complex reaches equilibrium more rapidlyat a temperature of F., the reaction is not as complete at thistemperature levely as it is at temperatures of 60 to 70 F.vkAccordingly, for higher recovery yieldsof isoprene, I prefer to employthe lower temperature range and to effect the extraction of the2-methy1butenes in the presence of adequate agitation.

In effecting the extraction step, a 65 weight-percent solution ofsulfuric acid flows through line 10 at about 50 F. into extractor 8. Therising mixture of straight and branched chain oleiins, and parainscontacts the descending stream of sulfuric acid which selectively,extracts the branched oleins. The temperatureat which this operation iscarried out is preferably maintained at about 60 F. Refrigeration sordinarily requiredvto hold the extracton zone at this level. Y l YRaiiinate, which consists chieily of paratlnsand straight` o chainoleiinsv, is taken overhead through line-1 2 and added Patented May 23,196;'1v

to the feed stream in line 2. The extract from the extraction step iswithdrawn through line 14 and consists essentially of the aforesaidcomplex of 2-methylbutene-1 and Z-methylbutene-Z. -In order torliberatevthese olens from sulfuric acid,.the-extract` in line; 14 is diluted,preferably to` a.sulfuric.acid1 concentration of about 45 weightpercent, by the addition of aqueous distillate from a sub-- sequentoperationinline 16. Additionalwater, if needed, maybe added to thesystem.r through line 18. This diluted mixture is then passed intostrippingcolumn 2.0`

reaches about 65 weight percent. This operation is generally carried outat a bottoms temperature of about 310i" F. The resulting concentratedacid is then withdrawn. through line 10, cooled to about 50 F. andrecycled toextractor 8.

The stream of Z-methylbutenes in line 22 is then sent to a seconddehydrogenation unit` 28 where these olefins arev converted intoisoprene. One of the outstanding features of my invention resides in thefact that I am able, by the previous steps outlined above, to supply afeed to said. second dehydrogenation unit consisting substantiallyentirely of components whichv can be readily converted into isoprene. Bythe use of a feed having such purity forA this particulardehydrogenation stage, the possibility of undesirable side reactions, socharacteristic of prior art processes, is held to a minimum.

The catalyst employed in dehydrogenation unit 28 may be any of a numberof well-known materials used for that purpose and which have beenpreviously recited. Chromia on alumina is typical of the catalyst whichmay be suitably employed at this stage. The temperature ernployed inthis second dehydrogenation stage may range from about 1,000."l to about1,200? F.; however,.the preferred range may be quite narrow, typically1,050 to 1,075,a F. Operating pressures are preferably maintained fromaboutZ; to about 4 p.s.i.a.

Spacev velocities employedV within, the temperature ranges specified;for bothdehydrogenation units 4 and 28 may range from about 20010. about2,000'volumesof `gas per volumegofcatalyst per hour.

The'mixture obtained from dehydrogenation unit 28 is withdrawnthrough`line 30, cooledV and transferred to fractionating column 32 operatingiat an overhead temperature of about 94 F., whereby isoprene andV lowerboiling. components; are taken oi-vialine. 34. A portion ofthedistillate in line 34 is returned to column 32 as.

andis recycledto dehydrogenation unit 28 Via lines 40 and 22.

Y As isJ well-known tothe art, the catalyst beds employedin-bothdehydrogenation stepsmust beY periodically regeneratedby-successive treatments with steam, air and steam, after which theproduction cycle may `be resumed.

TheY catalyst used in either of'4 thedehydrogenation steps required maybe in" the form of'xedor'fluid beds. Thus; in thecase of 1a tixed'bed,several alternativeV methodsVK of operation may' be usedin the catalyst"zone, for

4 example, alternative methods of distributing the catalyst in thereaction zone may be employed. Thus, the catalysts in dehydrogenationand isomerization unit 4 may be distributed in alternate layersthroughout the depth of the bed or the two 'catalysts may be physicallymixed so that uniform distribution of both of them is obtainedthroughout the zone. On the other hand, it may be deg sired todistribute the dehydrogenating oxide, suchl as chromia, upon theisomerizing catalyst as a support. It is seen, therefore, that variousmethods may be used for dispersing both catalysts throughout thereaction zone in unit 4.

In the event a fluidV bed is employed, either a mixture of powdereddehydrogenation catalyst with powdered isomerization catalyst may beused, or the dehydrogenating oxide may be distributed upon thesilica-alumina hydrogel isomerizing catalyst, thereby producing acatalyst which will effect both reactions.

The catalyst used in dehydrogenation unit 28 is, of course, a singlepurpose catalyst, i.e., dehydrogenation, and likewise may be used in theform of either fixed or uid beds.

The process of my invention may be further illustrated by the followingexample:

Example A `C5 natural gasoline fraction containing approximately 50.percent normal paraiilns and about 50 percent branched chain parains, issubjected to simultaneousv dehydrogenation and isomerization conditionslby contacting the vapors of this feed with a composite catalyst ofchromia-alumina and platinum oxide at a temperature of about.1,000 F. atan absolute pressure of about 3 p.s.i. and at a space Velocity of about500 volumes of gaseous feed per volume of catalyst per hour, measured atstandard conditions. The product resulting from this treatment is thensubjected to countercurrent extraction with a 65 weight percent sulfuricacid solution at a ,temperature of about 60 F. The raffinate from thisextraction step is recycled to the feed going to the above-mentionedd'ehydrogenation-isomerization step. The extract thus obtained isdiluted with water to give a sulfuric acid concentration of about 45weight percent and the resulting diluted mixture subjected to astripping operation at about F., whereby the Z-methyllbutenes, combinedwith the sulfuric acid during the extraction step, are liberated andtaken overhead. The dilute sulfuric acid is concentrated to about 65`percent strength, cooled to about 50 F, and recycled to the aforesaidacid extraction step. The Z-methylbutenes, obtained as a result ofthepreviously mentioned acidstripping operation, are then sent to a secondstage dehydrogenation operation where the gas velocity and' pressureareapproximately t-he same as those employed in the aforementioneddehydrogenation and isomerization step. The catalyst used at thisstageis primarily a dehydrogenation` catalyst, i.e chromia-alumina. 'YThe temperature used in this second dehydrogenation operation, however,is somewhat higher, being of the order of 'about 1,0509 F. VTheresulting effluent from the last-mentioned dehydrogenation step is thensent to a fractionation system Where product isoprene is recovered in'high yield. Mon'ooleiins separated during the isoprene recovery stepare recycled and combined with the Z-methylbutene feed going to thesecond dehydrogenation stage.

I claim:

1. In a process for the manufacture of isoprene from a C5 parainhydrocarbon by dehydrogenation ofthe latter, the improvementwhichcomprises introducing a mixture consisting essentially of branched andVstraightv chain C5 paraffin hydrocarbons into a reaction zone underconditions'favorable to the dehydrogenation ofsaid hydrocarbons and tothe isomerization of the olefins resulting therefrom, contacting saidmixture in said zone with` a composite mass 'consistingVY essentiallyofi aV ohromiacontaining d'ehydrogenation-catalyst and platinum-derived1 LA JM...

isomerization catalyst, withdrawing a product mixture containing a2-methylbutene from said zone, recovering from said product mixture byextraction with an aqueous sulfuric acid solution an extract streamconsisting essentially of a 2-methylbutene and a raffinate streamconsisting chiefly of parafiins and straight chain olens, recycling saidrainate stream to said dehydrogenation-isomcrization reaction zone nextbringing said extract stream into contact with a chromia-containingdehydrogenation catalyst in a second reaction zone under conditionsfavorable for the dehydrogenation of said 2-methylbutene to isoprene,withdrawing the resulting product mixture containing isoprene and'mono-olens, recovering isoprene thererom, and recycling said mono-olensto said second reaction zone.

y2. A process for the manufacture of isoprene comprising contacting a-feed mixture consisting essentially of isopentane and normal pentane ina iirst reaction zone with la composite catalyst consisting of achromia-alumina dehydrogenation catalyst and a platinum-derivedisomerization catalyst under conditions to producea rst product mixturecontaining pentenes and including 2-methylbutene, withdrawing from saidrst reaction zone said rst product mixture, subjecting said iirstproduct mixture to extraction with an aqueous sulfuric acid solution,liberating said 2-methy1butene from the resulting extract, recycling theresulting rainate to said lirst reaction zone, and thereafter contactingsaid liberated 2-methylbutene with a chromia-alumina dehydrogenationcatalyst under conditions to produce a second product mixture containingisoprene, recovering isoprene from said second product mixture andrecycling the remainder of said second product mixture to said secondreaction zone.

3. A process for the manufacture of isoprene comprising contacting afeed mixture consisting essentially of isopentane and normal pentane ina first reaction zone with a composite chromi'a-alumina dehydrogenationcatalyst and a platinum oxide isomerization catalyst at a temperatureranging from about 900 F., to about 1100 F. at a pressure of from about2 to about 4 p.s.i.a., at a space velocity ranging from about 200 toabout 2000 volumes of gas per volume of catalyst per hour, withdrawingfrom said clirst reaction zone a -first product mixture containing aZ-methylbutene, subjecting said product mixture to extraction with anaqueous 60-75% (wt.) sulfuric acid solution, at a temperature of about-100 F. liberating a Z-methylbutene from the resulting extract,recycling the resulting rafinate to said feed mixture, thereaftercontacting said liberated Z-methylbutene with a dehydrogenation catalystconsisting essentially of chromia-alumina in a second reaction zone at atemperature between about 1000 F. and 1200 F. and a pressure of about 2to 4 p.s.i.a. and a space Velocity of about 200 to about 2000 volumes ofgas per volume of catalyst per hour to dehydrogenate said 2-methylbuteneto isoprene, withdrawing and fractionating the resulting second productmixture from said second reaction zone to recover isoprene therefrom andrecycling the remainder of said second product mixture to said secondreaction zone.

References Cited in the le of this patent UNITED STATES PATENTS2,346,657 Bloch et al Apr. 18, 1944 2,391,160 Hillman et al. Dec. 18,1945 2,394,625 Matuszak Feb. 12, 1946 2,421,506 Jones June 3, 19472,515,006 Hudson July 11, 1950 2,831,908 Starnes et al Apr. 22, 1958

1. IN A PROCESS FOR THE MANUFACTURE OF ISOPRENE FROM A C5 PARAFFINHYDROCARBON BY DEHYDROGENATION OF THE LATTER, THE IMPROVEMENT WHICHCOMPRISES INTRODUCING A MIXTURE CONSISTING ESSENTIALLY OF BRANCHED ANDSTRAIGHT CHAIN C5 PARAFFIN HYDROCARBONS INTO A REACTION ZONE UNDERCONDITIONS FAVORABLE TO THE DEHYDROGENATION OF SAID HYDROCARBONS AND TOTHE ISOMERIZATION OF THE OLEFINS RESULTING THEREFROM, CONTACTING SAIDMIXTURE IN SAID ZONE WITH A COMPOSITE MASS CONSISTING ESSENTIALLY OF ACHROMIACONTAINING DEHYDROGENATION CATALYST AND PLATINUM-DERIVEDISOMERIZATION CATALYST, WITHDRAWING A PRODUCT MIXTURE CONTAINING A2-METHYLBUTENE FROM SAID ZONE, RECOVERING FROM SAID PRODUCT MIXTURE BYEXTRACTION WITH AN AQEOUS SULFURIC ACID SOLUTION AN EXTRACT STREAMCONSISTING ESSENTIALLY OF A 2-METHYLBUTENE SAID A RAFFINATE STREAMCONSISTING CHIEFLY OF PARAFFINS AND STRAIGHT CHAIN OLEFINS, RECYCLINGSAID RAFFINATE STREAM TO SAID DEHYDROGENATION-ISOMERIZATION REACTIONZONE NEXT BRINGING SAID EXTRACT STREAM INTO CONTACT WITH ACHROMIA-CONTAINING DEHYDROGENATION CATALYST IN A SECOND REACTION ZONEUNDER CONDITIONS FAVORABLE FOR THE DEHYDROGENATION OF SAID2-METHYLBUTENE TO ISOPRENE, WITHDRAWING THE RESULTING PRODUCT MIXTURECONTAINING ISOPRENE AND MONO-OLEFINS, RECOVERING ISOPRENE THEREFROM, ANDRECYCLING SAID MONO-OLEFINS TO SAID SECOND REACTION ZONE.